1. Field of the Invention
The invention relates generally to digital transmission systems and particularly to ones that control a clock on the basis of the timing of a received digital message in order to be able to demodulate the message.
2. Description of the Prior Art
In digital transmissions the recurrence frequency of the received information is known, but not the phase. The phase is partly a function of the phase at the origin of each binary element (called a bit herein) and depends partly on the conditions of propagation. To demodulate the digital message received, a clock whose phase is controlled by the timing of the received digital message is required. This control must take place when the beginning of the message (the message being made up of indexing bits followed by information bits) arrives. It should be noted that the indexing bits serve to determine the first information bit. The message is preceded by a signal, called a "roll", containing logical sequences of "0" and "1".
The drawback of this procedure relates to short and repetitive messages in which the time allocated to the indexing bits correspondingly reduces the time available for the transmission of information bits.
The purpose of the present invention is to eliminate this drawback.
One type of prior art synchronization apparatus is shown and described in French Pat. No. 1,426,996 which is made part of the present application.